1. Field of the Invention
This invention is related to an single-phased electromagnetic actuator having at least one stable position in the absence of current, and including a movable member and a stator structure, wherein the movable member includes a portion which is magnetized normal to the direction of displacement, and the thickness of the magnetized portion is small compared to the other dimensions thereof. The stator structure includes two magnetic circuits made of a very high magnetic permeability material and has an air gap in which is arranged at least one portion of the magnetized portion The stator structure includes at least a field electrical coil coupled with the magnetic circuit.
2. Discussion of Background
The International Patent Application CH-87-00063 filed Jun. 1, 1987 under priority of Switzerland Patent Application 222886 filed Jun. 2, 1986, discloses an electromagnetic actuating apparatus allowing the movable member to move a limited distance under a substantially constant force. This apparatus is particularly intended to actuate the read-write head of a rotative disk memory such as a hard disk, a magnetic disk or an optical disk.
The actuator apparatus involves several major drawbacks. First of all, the closing of the magnetic circuit is carried out by magnetic joints arranged either behind the stator structure, namely, normal to the displacement path of the movable member, or in planes parallel to the displacement path of the movable member. In both cases, the overall size of the stator structure is significantly increased, which can be prejudicial to a number of micromechanical applications where volume is critical as is particularly the case in hard disk units. Such actuators are particularly implemented in hard disk units for the displacement of the read head.
Still increasing requirements for miniaturization do not allow to use an actuator including a closed stator structure. On the other hand, in many applications the movable member is intended to be kept in a steady position in the absence of an energizing current of the electrical coil. The locking force of the movable member, however, in this steady position must be controlled so as to avoid an excessive sticking which would disturb the actuator's normal operation. This characteristic is for instance of value when the actuator is intended to control a plotting table pen. In this case, the plotting pen shall preferably remain, in the absence of current, in a raised position, i.e., separated from the paper after the current has moved the pen in the high position.
In the apparatus known in the prior art, a spring having a defined force resulting in a very little stiffness is used to obtain such a result. This solution is, however, not entirely satisfactory because the spring force acts in a permanent way. Hence, this will disturb the force exerted by the electromagnetic actuator. On the other hand, the differences in physical characteristics of a family of theoretically identical springs causes variation in the writing force, exceeding the allowable tolerances in the case where the writing force is poor. By way of example, if the writing force advised for a pen is about 0.5 newton, and if the spring is calibrated at 0.8 newton with a tolerance of + or -0.1 newton, such tolerance will generate a variation of + or - 20% on the writing force.